A vitrectomy specimen from a diabetic patient was studied by light and electron microscopy using myosin subfragment 1 to decorate and identify actin filaments. The patient had proliferative diabetic retinopathy, a shrinking fibrovascular preretinal membrane associated with retraction of the thickened posterior hyaloid, and a localized traction retinal detachment. The fibrovascular tissue comprised normal mature collagen, few cells, and occasional blood vessels. The cells contained numerous thick bundles of the contractile protein, actin. We suggest that actin may have been involved in the contraction phenomena observed clinically and that membrane contraction might be blocked by pharmacologic treatment.